Acoustic wave sensors such as, for example SAW and/or BAW based sensors, can be utilized in a variety of sensing applications, such as, temperature and/or pressure sensing. Such surface wave sensors can also be utilized to detect the presence of substances, such as chemicals. Surface acoustic wave devices are typically fabricated using photolithographic techniques with comb-like interdigital transducers placed on a piezoelectric material. Surface acoustic wave devices may have either a delay line or a resonator configuration. One application where SAW sensors have been utilized effectively involves pressure and/or temperature sensing of vehicle tires.
Such sensors generally communicate with a vehicle so that the sensed pressure can be displayed to the operator when the vehicle is moving, i.e. the wheel rotating relative to the body of the vehicle. Such devices are generally relatively complex and expensive or alternatively are not particularly robust. One type of sensing system utilized in automobiles is the TPMS (Tire Pressure Monitoring System), which incorporates a sensor that is fixed to a body and hence a rotating electrical contact between the rotating wheel and the chassis is not required. In a TPMS application, a sensor rod is deflected by contact with the tire sidewall when the sidewall of the tire is deformed. This system provides an indication of low tire pressure, but is also not very robust. For example mud or other debris on the wheels may cause faulty readings. Furthermore, this system provides an indication only when the tire pressure is reduced significantly as is necessary for significant tire bulge to occur. Clearly such a system simply cannot provide a reading of actual tire pressure.
The majority of prior art TPMS systems require batteries to transmit readings from air pressure and temperature sensors. The batteries have a limited lifespan and suffer from impaired performance under the temperature conditions often experienced by automotive components, thereby reducing the reliability of such systems. In addition, batteries contain chemicals that can have an adverse impact on the environment. Further, the weight of the battery itself can cause distortion of tire shape at high speeds, causing loss of air pressure and resultant safety problems. Hence, a battery-less system is more desirable.
Tire pressure sensors have been implemented based on the assembly of a configuration in which a SAW die floats on a base to which the SAW die is eventually wire bonded. The SAW die is highly sensitive to small stresses and even displacements involving microscopic dimensions such as nanometers, micrometers etc., which can easily occur either due to CTE (Coefficient of Thermal Expansion), mismatch of die, packaging materials and adhesive utilized to secure the die to a supporting structure at different temperatures or due to time-dependent adhesive properties at a constant temperature that interact with the location of that adhesive with respect to the die and the die supporting structure. These issues can result in sensor drift over a period of time at different temperatures resulting in inaccurate and unreliable operations for a given sensing application.
Referring to FIG. 1, a side view of a SAW-based sensor 100 utilizing a prior art die-attachment method is illustrated. The SAW-based sensor 100 utilizes four dots of a relatively soft adhesive 110 for attaching the die 130 to the die supporting base structure 140. The adhesive 110 is then cured to complete the process. The soft adhesive 110 offers a relatively lower Young's modulus and lower yield strength after curing than a hard or rigid bond adhesive and possesses a time dependent property (e.g. visco-elastic creep) or a combination of properties, which results in a gradual stress on the SAW device 100 resulting in a change (drift) of output over a period of time. These adhesive related changes can result in gradual stress effects on the SAW die 130 at a given temperature. The SAW sensor 100 is sensitive to these stress changes, resulting in an output drift from the sensor 100 and poor data.
Referring to FIG. 2, a top view of the SAW-based sensor 100 utilizing a prior art die-attachment method is illustrated. The SAW-based sensor 100 includes four dots of soft adhesive 110, die 130, a die supporting base structure 140 and a die supporting ledge 150. The soft adhesive dots 110 as depicted in FIG. 1 exert a gradual stress on the die 130 primarily from the sides of the sensor 100. Such a configuration causes the sensor 100 to drift over a period of time resulting in inaccurate and unreliable sensing operations and results.
Referring to FIG. 3, a graph 200 of sensor drift versus time utilizing soft adhesive is illustrated. As shown in graph 200 of FIG. 3, the sensor drift is high when a soft adhesive is utilized for attaching the die 130 to the die supporting base 140. Graph 200 generally plots frequency drift data from initial readings for three resonators (i.e., TSAW, RSAW, and PSAW) and their difference frequencies Fp and Ft which are utilized to calculate pressure and temperature.
Based on the foregoing it is believed that a need exists for an improved design, which can incorporate a rigid bond adhesive for die attachment to a supporting base structure for sensing applications for the achievement of enhanced sensor performance. It is believed that by utilizing the sensor packaging method described in greater detail herein, stress effects in the resulting SAW sensor device can be eliminated.